1. Field of the Invention
The present invention relates to a management system of monitor wafers, and more particularly, to a management system capable of automatically computing a reasonable inventory quantity of monitor wafers and automatically checking whether the inventory quantity is within a safety stock quantity or not.
2. Description of the Prior Art
In order to control the quality of integrated circuits, a so-called monitor wafer is used to ensure the quality of the wafers made in one wafer manufacturing process. That is, upon the completion of a wafer manufacturing process, the monitor wafers would be sent to the quality control department for quality testing; and according to the testing result, the operators decide whether the next manufacturing process or the present process should be performed on these wafers. However, the used monitor wafers have to go through a series of recycling processes before they can go back to the wafer production process. Besides, the monitor wafers used in different wafer manufacturing processes can substitute each other, under the condition that a wafer in one wafer manufacturing process has to go through a series of pretreatment processes before it can be used to replace the monitor wafer used in a different wafer manufacturing process. For example, the pretreatment processes need to find out the types and the quantities of the substitute monitor wafers that can be assigned to replace certain kind of monitor wafers. According to the demand quantity of the monitor wafers to be replaced and the available quantity of the substitute monitor wafers, certain substitute monitor wafers are assigned to replace the monitor wafers used in a different wafer manufacturing process.
A conventional management method of monitor wafers is to store the monitor wafers in a storage center. The operators have to inform the storage center of their demand quantities of the monitor wafers and then get the monitor wafers from the storage center. In addition, the monitor wafers passed predetermined manufacturing processes and tests are collected in a recycle box and sent back to the storage center so as to take recycling processes. In this case, the storage center has to periodically calculate an inventory quantity of the monitor wafers and check whether the inventory quantity can satisfy the demand quantities from the manufacturing processes and maintain a safety stock. If the inventory quantity of the monitor wafers is found less than the quantity of the safety stock, the storage center has to buy new monitor wafers to avoid affecting the wafer production processes.
However, the conventional management method of monitor wafers cannot calculate an accurate value for the inventory quantity of the monitor wafers. For example, when the storage center finds that the inventory quantity of the monitor wafers is going to be less than the quantity of the safety stock, it may determine to order new monitor wafers to replenish the stock. Sometimes when the storage center-has just finished the replenish procedures or the store center is executing the replenish procedures, the used monitor wafers or the recycled monitor wafers are sent back to the storage center too. Since these used monitor wafers or recycled monitor wafers are enough to satisfy the quantity of the safety stock, the new monitor wafers only bring excess stock and unnecessary costs to the storage center instead. Because it is difficult of using the conventional management method to get enough information of the in-line monitor wafers, such as the in-line wafer conditions and the wafer quantities, the storage center cannot distinguish the qualities of the recycled monitor wafers or estimate a reasonable processing time required for these recycled monitor wafers to undergo the recycling processes. In other words, it becomes very difficult for the storage center to predict a reasonable inventory quantity of the monitor wafers or even to simplify the recycling processes of some of the recycled monitor wafers according to their wafer conditions, thus resulting in waste of production costs.